feat(utils): define file config (#13283)

** What
 - Allow auto-loaded Medusa files to export a config object.
 - Currently supports isDisabled to control loading.
 - new instance `FeatureFlag` exported by `@medusajs/framework/utils`
 - `feature-flags` is now a supported folder for medusa projects, modules, providers and plugins. They will be loaded and added to `FeatureFlag`

** Why
 - Enables conditional loading of routes, migrations, jobs, subscribers, workflows, and other files based on feature flags.

```ts
// /src/feature-flags

import { FlagSettings } from "@medusajs/framework/feature-flags"

const CustomFeatureFlag: FlagSettings = {
  key: "custom_feature",
  default_val: false,
  env_key: "FF_MY_CUSTOM_FEATURE",
  description: "Enable xyz",
}

export default CustomFeatureFlag
```

```ts
// /src/modules/my-custom-module/migration/Migration20250822135845.ts

import { FeatureFlag } from "@medusajs/framework/utils"

export class Migration20250822135845 extends Migration {
  override async up(){ }
  override async down(){ }
}

defineFileConfig({
  isDisabled: () => !FeatureFlag.isFeatureEnabled("custom_feature")
})
```

packages/core/utils/src/migrations/index.ts

@@ -9,6 +9,7 @@ import { EventEmitter } from "events"
 import { access, mkdir, rename, writeFile } from "fs/promises"
 import { dirname, join } from "path"
 import { readDir } from "../common"
+import { CustomDBMigrator } from "../dal/mikro-orm/custom-db-migrator"
 
 /**
  * Events emitted by the migrations class
@@ -50,6 +51,7 @@ export class Migrations extends EventEmitter<MigrationsEvents> {
           ...this.#configOrConnection.migrations,
           silent: true,
         },
+        extensions: [CustomDBMigrator],
       })
     )
   }